1. Field of the Invention
The invention pertains to photodetectors.
2. Art Background
A photodetector (for the purposes of this invention) is a device which undergoes a detectable, internal change in its physical or chemical properties, e.g., an internal change in its mechanical, electrical, electronic, or optical properties, when exposed to electromagnetic radiation, and is a device which includes a means, e.g., electrodes, for detecting this change. Thus, for example, a photovoltaic device is a photodetector because such a device undergoes an internal change in its electrical properties, i.e., a current is induced within the device, when the device is exposed to electromagnetic radiation, and the device typically includes electrodes for detecting the current.
Two criteria which are often used to evaluate the usefulness of a photodetector for a particular purpose are its responsivity and operating speed. The former involves the magnitude of the change induced in response to the incident electromagnetic radiation, and the latter involves the time lag between a variation in the intensity of the incident electromagnetic radiation and the corresponding detected variation in the induced internal change.
The desire in many applications to have high responsivity and high speed imposes conflicting constraints on a photodetector configuration. Generally, to attain high responsivity, a relatively large amount of electromagnetic radiation absorbing material is required to produce a relatively large induced internal change, i.e., a large device structure is desirable. On the other hand, generally to attain high speeds, a small device structure should be employed. A small device decreases internal capacitance and/or internal change propagation distances and thus increases speed.
The conflicting constraints imposed on photodetectors which must exhibit both high responsivities and high operating speeds is exemplified by the conflicting physical constraints imposed on metal-isulator-metal (MIM) photodetectors required to have both high operating speeds and high responsivities. An MIM photodetector typically includes a layer of electrically insulating material, e.g., a metal oxide or a semi-insulator, sandwiched between two electrodes, e.g., two flat, smooth layers of metal. When a metal oxide is employed, the photodetector is denominated an MOM device, e.g., A1-A1.sub.2 0.sub.3 -Ag. When a semi-insulator is employed together with one transparent (at least 10 percent of the incident radiation is transmitted) electrode, the photodetector is denominated an MSM device, e.g., A1-amorphous Si-A1. A voltage difference (typically 1 to 2 volts) is applied across the electrodes during the operation of the photodetector.
Electromagnetic radiation incident on an electrode of an MOM device is largely reflected and to a much lesser extent is coupled into the device via the excitation of electrons in one or both of the electrodes. These excited electrons then tunnel across the oxide layer to the other electrode. The time required for these electrons to traverse the oxide layer (typically 50 Angstroms thick) is of the order of 10.sup.-14 seconds and thus, in principle, these MOM devies are capable of operating at very high operating speeds limited typically by device capacitance. However, the quantum efficiencies (the number of electrons produced within the MOM device per photon of incident radiation) of these devices is extremely low, typically about 10.sup.-4, because only a relatively small amount of the incident radiation is actually coupled into the device. Thus these MOM devices are unattractive for many practical applications.
When electromagnetic radiation impinges the transparent electrode of an MSM device, it is believed that a portion of the transmitted radiation is absorbed by the semi-insulating material where electron-hole pairs are produced. The electrons and holes drift through the semi-insulating material, to one or the other of the electrodes, under the influence of a voltage difference (typically 1-2 volts) applied across the electrodes during the operation of the device, to define a detectable current. Like the MOM devices, the MSM devices also exhibit high operating speeds because, for example, if the semi-insulating layer is only about 100 Angstroms thick, then the charge carriers only require a time interval of the order of 10.sup.-12 seconds to drift from one side of the semi-insulating layer to the other side. These MSM devices also exhibit low quantum efficiencies (typically about 10.sup.-3), again because only a relatively small amount of the incident radiation is coupled into the device, i.e., is absorbed by the semi-insulating layer to produce electron-hole pairs. The quantum efficiencies of the MSM devices are readily increased by increasing the thickness of the semi-insulating layer and thus increasing the absorption of transmitted photons. But the increased thickness of the semi-insulating layer increases propagation distance and thus reduces operating speed.
Consequently, those engaged in the development of photodetectors in general, and MIM photodetectors in particular, have long sought, thus far without success, techniques for enhancing the responsivities of photodetectors, which techniques do not require increases in the sizes of photodetectors.